In thermal ink jet printing, droplets of ink are selectively emitted from a plurality of drop ejectors in a printhead, in accordance with digital instructions, to create a desired image on a copy surface. The printhead typically comprises a linear array of ejectors for conveying the ink to the copy sheet. The printhead may move back and forth relative to a surface, for example to print characters, or the linear array may extend across the entire width of a copy sheet (e.g. a sheet of plain paper) moving relative to the printhead. The ejectors typically comprise capillary channels, or other ink passageways, which are connected to one or more common ink supply manifolds. Ink from the manifold is retained within each channel until, in response to an appropriate digital signal, the ink in the channel is rapidly heated and vaporized by a heating element disposed within the channel. This rapid vaporization of the ink creates a bubble which causes a quantity of ink to be ejected through the nozzle to the copy sheet. One patent showing the general configuration of a typical ink jet printhead is, for example, U.S. Pat. No. 4,774,530 to Hawkins.
When a quantity of ink, in the form of a droplet, is ejected from the ejector to a copy surface, the resulting spot becomes part of a desired image. Crucial to image quality in ink jet printing is a uniformity in spot size of a large number of droplets. If the volumes of droplets ejected from the printhead over the course of producing a single document are permitted to vary widely, this lack of uniformity will have noticeable effects on the quality of the image. Similarly, if volumes of droplets ejected from the printhead differ during subsequent printings of the same document, then printing stability cannot be maintained; this is particularly important in color printing. The most common and important cause of variance in the volume of droplets ejected from the printhead is variations in the temperature in the printhead over the course of use. The temperature of the liquid ink, before vaporization by the heating element, substantially affects both the density and the viscosity of the ink. These two ink properties substantially influence the resulting spot size on the copy surface. Control of temperature of the printhead, then, has long been of primary concern in the art.
In order to maintain a constant spot size from an ink jet printhead, various strategies have been attempted. One example is U.S. Pat. No. 4,899,180 to Elhatem et al., assigned to the assignee of the present application. In this patent the printhead has integrated into it a number of heater resistors and a temperature sensor which operate to heat the printhead to an optimum operating temperature, and maintain that temperature regardless of local temperature variations.
U.S. Pat. No. 4,791,435 to Smith et al. discloses an ink jet system wherein the temperature of the printhead is maintained by using the heating elements of the printhead not only for ejection of ink but for maintaining the temperature as well. The printhead temperature is compared to thermal models of the printhead to provide information for controlling the printhead temperature. At low temperature, low energy pulses are sent to each channel, or nozzle, below the voltage threshold which would cause a drop of ink to be ejected. Alternatively, the printhead is warmed by firing some droplets of ink into an external chamber or "spittoon," as opposed to the copy surface.
PCT application 90/10541 describes a printhead in which the heating cycle for the ink is divided into several partial cycles, only the last of which initiates bubble formation and ejection of a droplet. In this printhead, therefore, the liquid ink is first preheated to a preselected temperature, wherein the ink will have known volume and viscosity characteristics, so that the behavior of the ink will be predicatable at the time of firing.
PCT application 90/10540 discloses a printhead control system wherein the temperature of the liquid ink is compared with a predetermined threshold value, and if it exceeds this threshold value, the pulse energy (proportional to the square of the voltage to the heating element times the time duration of the pulse) is reduced. According to this patent, the pulse energy may be varied by controlling either the voltage, the pulse duration, or both.
U.S. Pat. No. 4,736,089 to Hair et al. discloses a thermal printhead (as opposed to an ink-jet printhead) wherein printhead temperature is sensed by a voltage generating diode on the printhead itself. A detected temperature of the printhead is used to establish a preselected reference level. Bi-stable means are coupled to the thermal printhead to print or not print at a given time. Control means are used to turn the bi-stable means on when the controlled voltage is less than the reference level related to the temperature, and turns the bi-stable means off when the controlled voltage exceeds the preselected reference level, thus causing the time duration of a voltage pulse to the thermal printing means to be dependent on temperature.
U.S. Pat. No. 4,980,702 to Kneezel discloses a thermal ink jet printhead wherein outputs from a temperature sensor in the printhead are compared to a high or low level temperature reference. If the sensed printhead temperature is below the reference value, power to the heater in the printhead is turned on. If the temperature sensed is too high, the heater is turned off. The printhead is configured so that the temperature sensor and heater in the printhead are in close proximity.
It is an object of the present invention to provide a system for controlling the spot size of droplets emitted from an ink jet printhead in response to changes in temperature.
It is another object of the present invention to provide such a system in which modifications to the system for various specific ink jet printing purposes may be easily incorporated.
It is another object of the present invention to provide such a system which controls spot size without requiring direct control of the temperature of the ink in the printhead.
Other objects will appear hereinafter.